Since Coturnix turned me on to this paper on snail chirality in PLoS (pdf), I had to sit down and learn something new this afternoon.

Chirality is a fascinating aspect of bilaterian morphology. We have characteristic asymmetries—differences between the left and right sides of our bodies—that are prescribed by genetic factors. Snails are particularly interesting examples because snail shells have an obvious handedness, with either a left-(sinistral) or right-handed (dextral) twist, and that handedness derives from the arrangement of cell divisions very early in development.

Sinistral (left) and dextral (right) snail shells.

The direction of rotation in snails is determined rather simply, by single mutations. As an additional twist, though, inheritance is by maternal effect. That is, the genes in question have to be expressed in the mother, but the phenotype appears in her progeny. One way to think of it is that the result of a maternal effect mutation is delayed one generation—the snail that has a mutation to cause a dextral twist will have a sinistral shell, but all of her children will be dextral.

Snail chirality is one of those things there are a great many papers in developmental biology about, and that's the direction from which I've always approached it—as an illuminating molecular and cellular process, part of the complex story of how genes are translated into form. The cool thing about this new paper by Davison et al. is that I had to stretch my brain a little bit and think about it from a population ecology perspective, too.

Here's the new fact I learned: dextral and sinistral snails have a hard time mating with each other. In some species with low-spired shells, it may be impossible to have cross-chiral matings. The paper did not give any of the juicy details of the coital gymnastics of snail mating (darn it), but think of it as one individual with a right-handed screw, and the other with a left-handed spiral socket, and you can imagine that there might be some difficulty coupling. This, of course, turns the asymmetry from an odd exercise in developmental biology into a potential isolating mechanism that could promote speciation, and that at the least would have interesting effects on the allele distribution in populations.

There I was, thinking once the evo-devo revolution came, we could just line up all the pop gen people against the wall and be done with them. But noooo…they're going to be useful after all.

The paper examines snails of the genus Euhadra in Japan. They are quite lovely little animals.

(A) E. quaesita from Sendai and (B) E. murayamai from Myojo-san (in cave); dextral (C) E. senck. senckenbergiana from Imajyo, (D) E. senck. amoriensis from Tamayama, (E) E. senck. amoriensis from Iide-san, and (F) E. senck. ibukicola from Mt. Fujiwara.

There are 22 species in the group, and 5 of them are sinistral, with the rest dextral. The question is whether speciation events can reasonably be traced back to changes in a single gene, whether this diverse assemblage can be explained by occasional mutations in chirality that split off new reproductively isolated groups.

Distributions of Four Euhadra Species on the Main Japanese Island of Honshu. E. senck. aomoriensis specimens that were used for DNA analysis were collected from Tamayama, Tsugaru, and Iide-san. E. murayamai is confined to a single mountain (Myojo-san). Sinistral species are shown in red and dextral species in blue.

To make a long and somewhat mathematical story short, the answer is no. There have to be other isolating mechanisms present to help out.

One observation is that if you are a newborn dextral snail in a population of sinistrals, you're going to have a much harder time finding a mate than your sinistral cousins. The more common your morphology, the more likely you are to find a compatible mate. This competitive advantage for the most common form will typically drive the population towards a single chirality.

There are, however, conditions under which it is good to be a weirdo. When two species of the same chirality overlap, it will be common for individuals of those two species to mate—which may be fun, but it's fruitless. If one species has a subpopulation with a different chirality, though, they may have an advantage. While they are only able to mate with conspecifics of the same handedness, they won't be wasting time and gametes on members of the other species. This is a phenomenon called character displacement, and could be an additional force for speciation.

In the simpler case where a single population has two chiral variants, though, chirality is insufficient in itself to isolate the two forms. With mathematical modeling, the authors showed that the separation will be incomplete because of gene flow, so the two types will reach an equilibrium, but outside of chance variations, one will not replace the other. The catch is the way maternal effects are delayed in the expression of their phenotype by a generation. That means that a sinistral snail can mate with a sinistral snail, and their progeny may be dextral, and able to breed with the dextral population. Similarly, some of those dextral snails will mate with other dextral snails, and produce progeny which are sinistral. Gene flow is slowed between the two subgroups, but it would require other phenomena, such as geographic separation, to complete the process.

Davison A, Chiba S, Barton NH, Clarke B (2005) Speciation and gene flow between snails of opposite chirality. PLoS Biol 3(9): e282.